Isochronous Cyclotron U--120M.

Isochronous cyclotron U-120M, the basic experimental facility of the Nuclear Physics Institute (NPI), is the only cyclotron in the Czech Republic. It has been operating since 1977. Due to its parameters, i.e. wide range of operating energies and quality of the beam, this cyclotron was used both for fundamental research and applications in order to meet demands and needs of experimental groups from the NPI and external Czech and Slovak customers.

The cyclotron can accelerate ions within the range of the mass to charge ratio A/q = 1 - 2.8. The present internal radial ion source is suitable for acceleration of H^+, D^+, 4-He^+2 and 3-He^+2. Maximum proton energy is 36 MeV, and the maximum energy for heavier ions is given by 40 q^2/A MeV . Currents of an internal beam of protons and deuterons can reach 100 microA, and for extracted beams 5 microA. Particles are extracted from the cyclotron chamber by means of a 3-section deflection electrostatic system to the entrance of the beam lines. This system transports and distributes extracted particles to experimental targets and chambers. An analysing magnet assures a minimum energy spread of the beam which is necessary for physical experiments.

The accelerator has been upgraded recently and further modernization is made in order to extend experimental possibilities for fundamental research as well as for applications.


Table 1. Parameters of the accelerated deuterons (D) and protons (P).

ACCELERATED PARTICLES                                          D           P

energy of injection, keV.......................................17         19
energy after acceleration, MeV.................................20         25
magnetic field in the cyclotron centre, T.....................  1.82       1.36
I_0 = beam current between two B-channels, microA...........    6         22
beam current on the inflector/I_0, %...........................90         80
buncher switched off:
  accelerated beam current (R=100mm)/I_0, %....................36          6.8
  accelerated beam current (R=150mm)/I_0, %....................19          5.2
  accelerated beam current (R=500mm)/I_0, %....................10          2.8
buncher switched on:
  accelerated beam current (R=500mm)/I_0, %....................40         12
  acceptance without inflection, mm.mrad......................459pi      398pi
  acceptance with inflection and acceleration,...
                                 mm.mrad......................126pi       65pi

Machine development.

System of axial injection.

An axial injection system is a basic precondition for acceleration of a wide spectrum of ions produced by external ion sources.

The first version of the axial injector was based on electrostatic elements (electrostatic spiral quadrupoles). Due to unsatisfactory optical parameters of these elements, we worked out a new version of the injector, which is based on electromagnetic elements (solenoids). and successfully tested in 1992. We measured the efficiency of an ion transfer from the external ion source including their catch into the accelerating process. Measurements were done with the beam of low intensity and emittance (see Table 1). Values varied from 12% to 40% [1].

The B - Channel.

The main focusing elements of the axial injection system are two electromagnetic channels, classical solenoids modified by an appropriate supplementation of the inner magnetic structure [2]. Experiments proved their ideal axial symmetry of the magnetic field of the solenoids and their extended acceptance. Due to those parameters, high transmission efficiency of ions through the whole injection line was reached. The B - channel was registered as a Czechoslovak patent.

Vacuum.

Negative ions or heavier ions with higher charge stage can be accelerated in the vacuum chamber and transported with low losses through beam lines provided that the residual gas pressure is sufficiently low. The vacuum has an essential influence on the beam losses, mainly in the acceleration chamber where the length of orbits can be hundreds of meters. From that reasoning, the whole vacuum system was renovated, diffusion pumps of the cyclotron were replaced by new Balzers ones, and all diffusion pumps of the beam lines were replaced by turbo-molecular pumps.

Some mechanical parts of the vacuum system were reconstructed. The whole modernization resulted in a significantly better vacuum. The pumping speed of the whole system increased and the whole operation is much simpler now. Reached residual gas pressure 1.5x10E-4 Pa is very close to the vacuum limit, which is given by the mechanical construction of the cyclotron vacuum system. Now the operating pressure is close to the value required for an effective acceleration of aforementioned ions.

Li--ion source

The accelerated beams of Li-ions with different degrees of ionization allow us realization of top experiments for fundamental and applied research.

The development of the Li-ion source, which is based on the duoplazmatron, is in its final stage. Intensity of heavier ions (i.e. 3-He^2+, 4-He^2+, 14-N^n ) was essentially improved by a new setting of geometry and magnetic field in the discharge chamber. Intensity of produced light ions (i.e. p, D^+) remained at a sufficient value. Nowadays, technology of production of Li ions (i.e. 6-Li^2+, 7-Li^2+, 6-Li^3+, 7-Li^3+) from solid lithium is being tested. Development of the Li--ion source will be finished at the end of 1994 and accelerated Li ions should be provided for our experimental groups.

Ion source for heavier ions.

In 1995, second stage of the development of the duoplazmatron will begin. The duoplazmatron will be converted into a double discharge chamber ion source, which would produce intensive beams of heavier atoms (e.g. N, C, O) with double and higher charge stage. The second discharge chamber has been manufactured and will be tested.

Acceleration of negative ions H^-, D^-.

In order to meet demands for high currents of extracted light ions, it is desirable to accelerate negative ions. Extraction of negative ions from the acceleration chamber is simpler and more efficient in comparison with the extraction of positive ions by a deflection system. Electrons are stripped in a carbon foil and the negative ions are converted into positive ions with 100% efficiency. The change of charge causes an opposite orientation of the Lorentz force vector and the ions are effectively extracted by the magnetic field of the cyclotron without any deflection system.

We upgraded and set into operation the Dubna model of internal radial ion source with a cold cathode for production of H^-, D^- ions in order to gain some experience with the acceleration of negative ions.

The computer code for calculations of stripped ion trajectories was generated and all the needed calculations were done on the basis of measurements of the cyclotron magnetic field.

Supporting mechanism of the stripping foil maintaining the accurate position of the foil, and its reproducible replacement was designed, manufactured and tested in 1994. We extracted proton beam of intensity 15 microA recently.

By the end of 1994 we will be able to irradiate targets with utilization of negative ions from the internal ion source. Connection of the CUSP ion source of high H^-, D^- currents to the axial injector and its routine operation is in progress and will be completed in December 1995.

Importance of this project can be summarized as follows:

Control system of the cyclotron.

The isochronous cyclotron U-120M with the system of axial injection, external ion-sources and the system of several beam lines, including the experimental facilities, and a complicated radio-technological complex have several hundreds of parameters to be monitored and controlled. Operative tuning and optimization of different regimes as well as monitoring and controlling of all the parameter settings during the cyclotron operation is a complex task that must be solved by a sophisticated computer control system.

The first part of the control system for the magnetic field of the cyclotron, based on mathematical modelling, was put into operation at the beginning of 1993. The model includes, besides others, phase motion of accelerated ions, transversal stability, and real possibility to form magnetic field by the main coil, 18 correction coils and 2 harmonic coils. The calculated values of currents are set up and controlled during operation by PC. Power supplies for magnetic coils have been upgraded. The aim was to essentially increase the reliability and exchangeability of individual blocks of power supplies and to make possible control of these power supplies by PCs. This reconstruction was necessary for realization of the first stage of the new control system - controlling of the magnetic field by computer.

Realization of the new control system will contribute to increased reliability of the cyclotron complex as well as reproducibility of beam parameters.

Applications.

Utilization of the cyclotron U-120M.

Table 2 shows the operating report of the cyclotron during period 1990-1993. The total number of operating hours was gradually reduced during this period, which was caused by the customers' financial situation. The number of operating hours in the first half of 1994 indicates that the downward tendency for use of the cyclotron has been stopped.

Table 2. Operating time report.

U-120M........................................1990.........1991........1992.......1993
fundamental research
Nuclear Spectroscopy Department of NPI        1240          456         367        160
Nuclear Reactions Department of NPI           1099          358         375        157
Neutron Physics Department of NPI                                                   68
Radioisotopes and Nuclear Biology Institute                                         74
Czech Technical University                                               10          6
Institute of Biophysics, Brno                                            31         23
applications
Nuclear Research Institute, Rez               1113          1190        951        928
State Research Institute of Materials                         46
Medical Army Institute                                                  194
target time                                   3768          2074       1734       1274

Production of radioisotopes.

The cyclotron is regularly used for production of radioisotopes 67-Ga, 201-Tl, and 111-In. Targets are irradiated directly inside the cyclotron chamber then processed in the Radioisotope Department of the Nuclear Research Institute and delivered to Czech hospitals. New system of monitoring of the beam distribution on the targets was developed so that production yield of radioisotopes has been increased and has been stabilized now.

We designed and installed a target which utilizes a secondary beam reflected from the surface of the first target for production of radioisotopes. Now, this target is used for a long term irradiation.

We also irradiated targets for production of 68-Ge, 207-Bi, 48-V as required by experimental groups of NPI.

[1] Z. Trejbal et al.: Proc. 13th Int. Conf. Cyclotrons and their Applications, Vancouver, Canada, July 6-10, 1992, p. 465.
[2] J. Stursa et al.: Proc. Europ. Part. Accel. Conf., Berlin, Germany, March 24-28, 1992.